1. Field of the Invention
The present invention relates to an induction heating roller apparatus and to a fixing apparatus and an image formation apparatus, which are provided with the fixing apparatus.
2. Description of the Prior Art
Heating rollers, which employ halogen lamps as heat sources, are used in the prior art to thermally fix a toner image. However, the halogen lamp heat sources are inefficient and require a large amount of power. Accordingly, a technique involving induction heating is being developed to solve such problems.
Japanese Patent Laid-Open No. 2000-215974 describes an exciting coil, which is arranged near a heated object. The exciting coil generates an induction current in the heated object, which is a magnetic heating roller. The exciting coil is formed by winding a coil in a planar manner along a curved surface of the heated object. A magnetic core is arranged along the curved surface or the exciting coil on the side opposite to the heated object at the longitudinal ends of the exciting coil (first prior art example).
Japanese Patent Laid-Open No. 2000-215971 describes an induction heating apparatus having a heating rotor, or heating roller, which generates heat by means of electromagnetic induction, and a magnetic flux generating means, which is arranged in the heating rotor. The magnetic flux generating means includes a magnetic core and an electromagnetic conversion coil, which is wound about the core. The magnetic core includes a core portion, about which the electromagnetic conversion coil is wound, and a magnetic flux induction core portion. The magnetic flux induction core portion, which has a magnetic gap between its distal ends, concentrates magnetic flux at part of a heating rotor rather than the core portion (second prior art example).
The first and second prior art examples employ a heating technique that uses eddy current loss (hereafter referred to as eddy current loss technique). Such heating technique works under the same principle as that applied to IH jars. The frequency of the high frequency employed in the eddy current loss technique is about 20 to 100 kHz.
In comparison, Japanese Patent Laid-Open No. 59-33787 describes a high frequency induction heating roller. The high frequency induction heating roller includes a cylindrical roller body, or heating roller, which is formed by a conductive member, a cylindrical bobbin, which is arranged in the roller body in concentricity with the roller body, and an induction coil, which is spirally wound about the periphery of the bobbin. When current flows through induction coil, the induction coil, which induces induction current in the roller body, is heated (third prior art example).
In the third prior art example, the cylindrical roller body functions as a secondary coil, which is a closed circuit, and the induction coil functions as a primary coil. This causes transformer coupling between the primary and secondary coils and induces a secondary voltage in the secondary coil of the cylindrical roller body. Based on the secondary voltage, a secondary current flows in the closed circuit of the secondary coil. This is a heating technique (hereafter referred to as a transformer technique) that heats a secondary resistor, which heats the cylindrical roller body. The transformer technique, which has a high stationary efficiency since its magnetic coupling is stronger than the eddy current loss technique, entirely heats the heating roller. Thus, the transformer technique is advantageous in that is simplifies the structure of a fixing apparatus in comparison to the first and second prior art examples. Further, when the operational frequency is 100 kHz or greater, and preferably a high frequency of 1 MHz or greater, the Q of the induction coil may be increased to increase the power transmission efficiency. This increases the total heating efficiency and reduces power consumption. Further, the heat capacity is much smaller than that of the eddy current loss technique. Accordingly, the transformer technique is preferable for increasing the speed of thermal fixing.
The inventors have invented a transformer coupling technique that efficiently heats the heating roller. In the transformer coupling technique, by forming a closed circuit, the secondary reactance of which is substantially equal to a secondary resistance of the heating roller that is air-core transformer coupled to an induction coil, the efficiency for transmitting power from the induction coil to the heating roller increases. This efficiently heats the heating roller. An application for a patent for this invention was applied for in Japanese Patent Application No. 2001-016335 by the present applicant. The invention reduces power consumption for induction heating of the heating roller and facilitates increasing the speed of thermal fixing.
In an image formation means, such as a copy machine or a printer, paper on which images are formed is selected from multiple sizes. To cope with such function, the heating area of the heating roller must be changed in accordance with the paper size.